Conventionally, methods (radiographic image acquisition methods) of obtaining a radiographic image of a target are widely used for industrial nondestructive inspection or medical diagnosis. In a radiographic image acquisition method, generally, a target is irradiated with radiation, and the intensity distribution of the radiation that has passed through the target is detected.
More specifically, the most general method of the radiographic image acquisition methods is as follows.
A so-called “phosphor screen” (or “intensifying screen”) which emits florescent light in accordance with irradiation of radiation is combined with a silver halide film. This combined structure is irradiated with radiation through a target. The phosphor screen converts the radiation into visible light so that a latent image of the target is formed on the silver halide film. The silver halide film on which the latent image of the target is formed is chemically processed. Accordingly, a visible image of the target (a radiographic image of the target) can be obtained on the silver halide film.
A radiographic image obtained by such a radiographic image acquisition method is a so-called analog photo and is used for radiodiagnosis or tests.
Use of computed radiographic apparatuses (to be referred to as “CR apparatuses” hereinafter) which use an imaging plate (to be referred to as an “IP” hereinafter) coated with a photostimulable phosphor is also starting.
The above-described CR apparatuses are digital radiographic apparatuses. However, since an image formation process for a read by secondary excitation is necessary, they cannot immediately display a radiographed image (radiographic image), like an analog photo.
In recent years, apparatuses have been developed, which acquire digital radiographic images by using, as an image reception means, a photoelectric conversion means (an imaging element such as a CCD) on which pixels including small photoelectric conversion elements and switching elements are arrayed in a matrix.
Such apparatuses have conventionally been disclosed as radiographic apparatuses in which a phosphor is stacked on a CCD or an amorphous silicon two-dimensional imaging element (e.g., U.S. Pat. Nos. 5,418,377, 5,396,072, 5,381,014, 5,132,539, and 4,810,881).
In a radiographic apparatus using a two-dimensional imaging element, normally, correction called FPN correction or white correction is executed before image formation. FPN correction (Fixed Pattern Noise correction) corrects noise generated due to a dark current in each element of the two-dimensional imaging element.
To execute FPN correction, normally, an FPN image without X-ray irradiation is acquired under the same driving conditions as in X-ray irradiation. The FPN image is subtracted from an X-ray image, thereby executing correction. White correction is also called gain correction and corrects the sensitivity difference between the elements of a two-dimensional imaging element (gain correction in this specification means white correction). To execute white correction, normally, subtraction processing for a radiographed image is executed by using an image (a white image) acquired by X-ray irradiation within a dose range with linearity. The corrected radiographic image is effective in, e.g., a medical scene with urgency because it can immediately be displayed within about 3 sec even when another image processing called QA processing is executed. Advantages of these digital radiographic apparatuses over the analog photo technology are filmless processing, an increase in acquired information amount by image processing, and database building.
Even in the above-described digital radiographic apparatuses using an imaging element of amorphous silicon or the like as an image reception means, an afterimage may remain in the image in accordance with an irradiation variation in pre-exposure, as in the radiographic apparatuses using a film or CR. FIG. 8 shows the concept of the time characteristic of an afterimage by a phosphor. An afterimage is generated by a phosphor afterglow. The ordinate is normalized on the basis of the X-ray dose of pre-exposure. As shown in FIG. 8, the phosphor afterglow attenuates over time. Then, the afterimage amount is almost constant. When the afterimage amount changes over time, as shown in FIG. 8, an afterimage remains on an image by a digital radiographic apparatus. This is because an imaging apparatus using a two-dimensional imaging element executes FPN correction.
The afterimage amount difference between the X-ray image acquisition mode and the FPN image acquisition mode remains as an afterimage in the FPN-corrected image. Except the phosphor afterglow, for example, the transfer residue in the imaging element can also generate an afterimage. The afterimages include not only the FPN afterimage (additive lag) but also a sensitivity afterimage (multiplicative lag). A sensitivity afterimage appears on an image when the light emission amount of the phosphor changes at the time of X-ray irradiation in accordance with its prior state, unlike FIG. 8. The sensitivity afterimage cannot be grasped without X-ray irradiation. Hence, the afterimage to be grasped is limited to the FPN afterimage.
To erase the afterimages, a method called optical reset in which the imaging element of amorphous silicon or the like is fully irradiated with visible light of an LED, a method of executing white radiography (calibration) before radiography, or a method of prolonging the sleep time of the sensor has conventionally been used.
However, always executing the prior-art afterimage erase methods requires time and labor. Especially, when no afterimage is present, these methods are executed only wastefully from the viewpoint of radiographic throughput. To omit the process, a means for grasping the presence/absence of an afterimage in advance is necessary. In the conventional analog apparatuses using a film or CR apparatuses, it is difficult to determine the presence/absence of an afterimage because of the film development time or IP read time of CR. In the radiographic apparatus using a two-dimensional imaging element, however, an image can instantaneously be acquired. Hence, it is technically possible to determine the presence/absence of an afterimage before radiographing an object.